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I'm trying to work out the logistics of a little fantasy-historical piece I'm working on.

Rome at the height of its power (c. 117 CE) has decided to invade a fantasy kingdom up north. It has a population of 1,750,000 and relatively advanced cross bows (the kind not used until much later by the French in medieval combat). This society however, doesn't have an organized army or militia on the level of Rome's, its population is mainly civilians, so the only chance in survival they have, is in their giant monstrosity of a wall which borders its entire territory, land and sea.

The wall is on average 49 meters high and typically 14 m thick and solid. It's 10 m wide atop, so wide enough for a track to rapidly carry supplies.
It's made of stone, brick, tamped earth, and some mixed-in limestone, and has a thinly polished quartz layer measuring 0.2 meter at the very top, to make it easily standable and maneuverable for those firing atop.

There are no gates - exit/entry is done through ropes only given and accessible from the top to citizens and diplomats prepared to leave.

This fantasy kingdom is only 156 km in diameter (area 19113 km² or 7380 mi²), but Rome wants it sieged and conquered. It's located in what would be modern day Germany and Belgium, with its ocean border facing the future United Kingdom.

Its sea coast is also bordered: 8 m of this wall is under water, so towering 41 m up from the sea.

Other than the technologically advanced crossbows and their massive wall, they possess no magic, no other superior technology or training.
They have a paltry militia of 11,250, including basic infantry and crossbow archers, but definitely not the sophisticated formations nor training of a Roman legion.

2) Since this is fictional, and I'm trying for different scenarios to write it more practically, what if the land size diameter was upped to 250 kilometers (area of 49,087 km² or 18,953 mi²) and rather than crossbows, this civilization had middle medieval technology, such as powdered muskets and cannons. Does this pose a greater difficulty for Roma invicta?

What difficulties would the Roman Empire be faced with having to scale such a construction? (How) would it be possible?

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    $\begingroup$ You call 112,500 men a paltry army? A Roman legion was about 5000 to 5500 men, and the Roman Empire only had around thirty of those from the time of Tiberius onward (a total of around 150,000 to 165,000 men for a colossal empire). And you're feeding this army with ancient agricultural techniques in a rather small area, at that! Seriously, I think you added an extra 0 or something here, especially given your description of this kingdom as not having any sort of organized army on the level of Rome despite having military numbers nearly equal to the entire Roman army in that small area. $\endgroup$ – Palarran Nov 15 '16 at 15:17
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    $\begingroup$ I thought this wall was insane, but I did some back of the napkin math and found its volume is in the ballpark of the Great Wall of China. This premise seems almost possible. $\endgroup$ – Cody Nov 15 '16 at 19:28
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    $\begingroup$ The problem here is that Rome wouldn't want to conquer it in the first place. Unless the cost of conquest < the benefit why would they bother? Julius Caesar would just have built another wall around it to keep them out of his domain. $\endgroup$ – user207421 Nov 15 '16 at 22:54
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    $\begingroup$ Is there a reason that you give the diameter of the country in km, but the area in mi^2 ? Km^2 would be more consistent... $\endgroup$ – Gerhard Nov 16 '16 at 8:52
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    $\begingroup$ @Servaes "The year is 50 BC. Gaul is entirely occupied by the Romans. Well, not entirely... One small village of indomitable Gauls still holds out against the invaders. And life is not easy for the Roman legionaries who garrrison the fortified camps of Totorum, Aquarium, Laudanum and Compendium....." $\endgroup$ – Maciej Piechotka Nov 17 '16 at 23:42

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It's going to depend a lot on the wealth and organization of the society within. A dedicated militia of 11,250 like you mention might be enough to patrol the wall and hold off any attackers long enough to call up the general militia, but that's pretty well got to be the main strategy. Anything else has them too spread out to not simply be overwhelmed by sheer weight of numbers. Allowing minimal overhead for management and leave time, you're probably not going to have more than 20 men/kilometer of wall, while the Romans get to choose the time and place and can throw an entire legion at that same section, if not multiple legions. Even if everyone comes running from 5 km in either direction, that's still only 200 men. Sufficient to slow them down, but the Romans could be armed with pointy sticks and climbing up the wall by forming a human pyramid and they'd win unless the defenders have either extremely rapid transport, or the ability to call for reinforcements from the general population.

So, if the society is wealthy enough that everyone has those fancy crossbows, and they are all reasonably good shots, then they can probably manage to repel a simple assault with ladders. Roman armor and formation fighting was formidable, but massed fire from crossbows of that power level would start to cause them problems, and probably at longer ranges than they could hope to throw their pila, especially when the defenders are atop a really high wall.

At that point, it comes down to a combination of deviousness and wealth. The Romans are going to need either a siege ramp or tunnels (preferably both) to get enough troops past the wall fast enough. Building those takes time and costs money. Countering those likewise costs money. The Romans will build catapults and siege towers to provide cover to those building the siegeworks, the defenders will build catapults, counter-tunnels, and moats to stop them, and the loser will be whoever runs out of resources first, or fails to notice one of the enemy's gambits.

Adding gunpowder to the mix makes a difference, but not in the way most people expect. The key thing about gunpowder is not that it makes the weapons more powerful. You could (and people have) build a crossbow just as powerful as a musket. A large trebuchet is arguably more powerful than early cannon. The thing about gunpowder is that it means that the source of energy for your weapons is no longer human and animal muscle. It can be stockpiled, and delivered in large quantities wherever and whenever it is needed, and it's not nearly so dependent on the physical fitness of individual soldiers.

Again though, it will come down to wealth. With gunpowder and cannon, your special militia might well be sufficient without having to call up the general militia. A single cannon with grapeshot or (if you up their tech a little bit) cannister rounds could easily provide a similar amount of aggregate firepower as an entire regiment of crossbowmen, plus a heavy catapult; and a good team of three gunners could keep up that rate of fire pretty much all day as long as their ammunition held out. So the question is, "how much ammunition do they have stockpiled, and how fast can they make more?"

The Roman legions were formidable, but keep in mind that, for the most part, they were attacking civilizations of a lower weapons technology level, and with lower military production capability. And even then, there were areas that gave them some trouble. A civilization with the infrastructure and resources necessary to produce late mediaeval crossbows in significant quantity could easily have sufficient resources at its disposal to make conquest by the Romans a foolhardy venture. Or, they might have built a few of the things, and then settled down to their nice, peaceful existence and nobody but the special militia even know which end of a sword is sharp anymore. That choice will make far more difference to the outcome than the power level of the individual weapons they use.

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  • $\begingroup$ Two small details which weren't over-explained in the Ask, the watch for the walls isn't necessarily made entirely from the militia, I envisioned a crew of few thousand extra non-militia sentry keeping watch and guarding the walls. Secondly, the top of the wall is wide enough to have a track that a cart can carry supplies and men along, and since it's 14 m wide it's not unreasonable for them to use horses atop the wall aswell. Still a hell of a distance to guard properly, but keep those in mind. $\endgroup$ – hankhoward Nov 16 '16 at 5:44
  • $\begingroup$ Trebuchets can throw around heavier ammo, but they aren't more powerful. You're not going to break a brick wall with a trebuchet. Look at how defenses developed once true cannons became common - short, solid walls of brick, with lots of dirt in front of them. That's what it takes to stop a cannon - even a small projectile travelling fast enough will bring a large stone (and to a lesser extent, brick) wall down, because they both rely on compression to hold together. Make a hole, and the whole height of the wall comes crashing down, and the higher the wall, the bigger the breach. $\endgroup$ – Luaan Nov 16 '16 at 11:20
  • $\begingroup$ @Luaan early cannon had nowhere near the power of the ones in use even a century later. The metallurgy to contain the pressure wasn't developed yet, and the ability to make consistently-sized projectiles wasn't either, so the barrels had to be conical rather than cylindrical, which wastes a lot of the power. Large trebuchets were more than capable of knocking down walls. The difference is they were shorter range and took a lot longer to load and aim, giving the defenders more time to return fire or to sally and attack the weapons. $\endgroup$ – Perkins Nov 16 '16 at 19:32
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    $\begingroup$ Why shoudl teh romands go up the wall as human pyramid? Why can't they use a siege tower or ladder/plank $\endgroup$ – BlueWizard Nov 18 '16 at 14:10
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    $\begingroup$ @JonasDralle It's not that it wouldn't be smarter to use a better strategy, it's that 200 men on top of the wall have no chance against the 10,000 men at the bottom (assuming the Romans only bother to send two legions.) Roman losses would be heavy, but they could fight until the guys on top were exhausted from killing them, and then boost each other up the wall and beat the defenders to death with sticks and rocks. Need more defenders or much higher tech. $\endgroup$ – Perkins Nov 21 '16 at 18:54
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Laying aside the 112,000 man army (which is monstrous), I'll address Rome and the wall.

Rome was entirely capable of working through almost any typical engineering problem involving such a wall. The thing about the Romans was that they were very patient. They would build a "camp" that might remain in place, working toward an objective for 20 or 30 years if need be. They would construct bridges over massive rivers that any other civilization would consider permanent, just to march an army across for a week, then destroy the bridge. Most people were in awe of their engineering prowess for very good reason.

Of course, a 49m wall wouldn't require anything close to that level of patience. They would be over that in less than a month.

Look up the siege of Masada, where a group of Jewish Zealots held out on top of a natural butte fortress whose low side was 90m up with almost totally vertical faces. The Romans took about a year, but they built a massive earthwork right up that face. The used slave labor to do it, and they moved enough earth to permanently change the shape of the landscape. During this time, multiple Roman camps were built to cover the construction. Each of these camps had overlapping fields of fire from onagers, ballista, and other types of Roman artillery. These artillery emplacements would keep up a continuous fire on the part of the wall over the ramp, to keep the defenders from seriously impeding their progress.

My guess is about a month to get over the wall you describe, but it might be a little longer depending heavily on the topography and geology of the location chosen for the breach. If they have better building materials available than they did in Israel's desert region (Masada), they could get things done faster, and if there were other problematic circumstances, it could take longer. The point is; once Rome decided to get over that wall, it would be gotten over inevitably. Your fantasy kingdom has no chance whatsoever.

Edit: A note on crossbows: The Romans were extremely disciplined and well trained, and they had a formation that they liked to use against enemies using ranged weaponry called the Testudo (tortoise). Each legionnaire would lock shields with the guy next to him, and the row behind the front would lock shields over the heads of the guys in front, and the entire century would move forward in perfect sync as a block of men. Now, crossbows are unlikely to penetrate too much through a legionnaire's shield (this can be debated, depending on the crossbow and the time period in which it was built), so worst case, you do some damage to a few guys' arms, but then they are close enough to first fire off their javelins, and then move in for close combat with short swords. Basically, their default organization would make crossbows NOT very helpful in an open field battle.

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    $\begingroup$ Considering the length of the wall, they can easily choose a good topography. Go romans! $\endgroup$ – PatJ Nov 15 '16 at 16:01
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    $\begingroup$ @hankhoward, one other thing: the term "Zealot" in English comes from these totally fearless, absolutely dedicated, and very effective rebel warriors during the Jewish uprising against Rome. They willingly died rather than be captured, so I would never put "Jewish Zealot" and "fleeing" in the same sentence. They never did. $\endgroup$ – JBiggs Nov 15 '16 at 16:31
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    $\begingroup$ +1 I heard high wall and Rome and though of Masada as well. Though if I remembered correctly the Romans relied more of shields and movable sheds to protect the workers on the ramp and less on sustained artillery fire. This has the advantage that the defenders and you have limited ammo. $\endgroup$ – sdrawkcabdear Nov 15 '16 at 18:37
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    $\begingroup$ At Masada, the Romans also took advantage of the Jews' unwillingness to harm the Jewish slaves who were being forced to build the ramp. $\endgroup$ – ruakh Nov 15 '16 at 18:42
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    $\begingroup$ "What difficulty would the Roman Emp-" "None!" Very well explained. +1. $\endgroup$ – Nij Nov 16 '16 at 7:05
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The Romans would mine the wall

The reason that the Romans had to scale the walls at Masada is because it was built on a rockface. Mining into a rockface is hard, though it could be done, slowly. Your wall is surrounding an entire kingdom. Therefore, I conclude that some point of the wall is built on nice, soft soil.

Polybius' Histories (Chapters 21.26-21.28, 21.28 has the mining) has an account of a mine and countermining operation in the seige of Ambracia in 189 BC. Simply put, if you can dig under the wall, you can simply cause the wall to collapse under its own weight and enter the breach.

That link is also some general good reading about how sieges went down in Roman times. Other good information can be found in Josephus' War of the Jews (Bk III, Ch 5, Roman Camps; Bk III, Ch 7, Seige of Gadara; Bk VII, Ch 8, Seige of Masada), the Tactics of Aelian (I can't find a free online version) and, of course, the granddaddy of them all Caesar's Gallic War which has seiges and camp building a-plenty.

A pre-Roman (4th century BC) writer who has perhaps the best descriptions of siege warfare of any of the above is Aeneas Tacticus.

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    $\begingroup$ @hankhoward The weight of the wall is what does the collapsing. Your wall is huge, thick, and heavy, and thus extra prone to collapsing. Even a solid piece of granite that size would have trouble supporting its weight over a 30 foot gap; a composite masonry structure would be even more susceptible. The length of the wall is what makes this tactic likely to succeed. If the Romans start digging in 3 or 4 different places, they are going to stretch the defenders very thin. $\endgroup$ – kingledion Nov 15 '16 at 18:19
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    $\begingroup$ ...and route a river into the wall. Let it be washed away. $\endgroup$ – Tony Ennis Nov 16 '16 at 2:10
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    $\begingroup$ @hatchet The psychological advantage "mwahaha, your mighty wall has fallen", plus building large enough tunnel to transport your troops, horse & supplies might take more time than just collapsing the wall $\endgroup$ – Martheen Nov 16 '16 at 7:59
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    $\begingroup$ @hatchet Collapsing the wall is easier. It takes a lot less digging for an opening of the same size - the wall breaks in a sort of wedge shaped pattern, and tends to tearn down large amounts of surrounding wall. $\endgroup$ – Luaan Nov 16 '16 at 10:36
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    $\begingroup$ @hatchet As Martheen noted, collapsing the wall makes a larger hole, but perhaps more importantly, it's a lot harder for the defenders to undo. Tunnels can be collapsed without being inside them, and those using the tunnel can't really prevent it. Also, if the wall comes down, the attackers outside it can cover those traversing the gap with bows, javelins, and the like, whereas those using a tunnel are on their own. Finally, the collapsed portion provides a way for the attackers to climb to engage the defenders on the wall, who will otherwise present a serious problem. $\endgroup$ – anaximander Nov 16 '16 at 14:43
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You seem to have a severe lack of understanding of what the Romans were capable of, and the scale of things 2000 years ago.

Logistics isn't what it used to be. The storage and preservation of food isn't what it used to be. Hygiene most definitely isn't what it used to be.

Food and hygiene

Packing that many people in such a small area in medieval times will result in starvation, and quite probably outbreaks of terrible diseases.

Agricultural productivity in medieval times was quite abysmal. Crops would fail all the time. Entire herds of animals would get sick and die. Thousands of people would die in a famine, and it would be no more than a sad event in a long list of similar ones. Disease was rampant, and only a small percentage of children - the strongest, and luckiest - would make it to adulthood, not to mention the many adults who would also die from something as simple as the flu.

You're packing a lot of people in a very small area. Sure, we do it today, but that's because we import food from half the world away via trains, trucks, etc. There are thousands of trucks driving into any given large city at any given time of day. If those food shipments stopped for even one day, you would notice shortages immediately. No more fresh bread. No more fresh fruit. Canned food stuffs would start flying off the shelves. We live in a very delicate balance in the big city.

Your 1.75 million people will be in bad shape in such a small area.

Your army and defenses

A wall 50 meters tall, 14 meters wide, and 156 km long is not just incredibly difficult to build, it's also a terrible defensive structure. You've essentially built yourself a fancy cage, and don't even know it yet. A wall needs to be constantly maintained (financial drain), as well as manned. Not to mention, that people need to get out, so you'll have lots of gates, which means weak points which can fall to the enemy.

Now on to your army. A modern country can only maintain an army around the size of 10% of the population before it starts running into economic woes. In war times it will rarely increase above 15%. Not sure how you're going to feed and equip 112,000 people, as these numbers were not encountered in medieval times. Some of the most famous battles of medieval Europe were fought with 5000 people or less on each side.

Even assuming you can keep your people fed and healthy however ...

You will lose

The Romans were the very embodiment of "improvise, adapt, and overcome". Their armies were not always larger than their enemies, but they were highly trained, highly organized, and well equipped. They were renowned for upgrading their armor, shields, and weapon designs based on tips and tricks picked up from conquered or enemy nations.

Your main weapon - crossbows - will only help you as long as their legions are further than arms length away. Furthermore, once the Romans come in contact with your troops and capture some of those weapons they will very quickly end up being used against you, and their armor will be improved to provide better resistance against them.

But I don't think this will even happen until after your nation has fallen. Here's how I think it would all go down:

The Romans will scout your walls, see where your defenders are more lightly concentrated. They will look for a weakly guarded gate, or a place where they can more easily approach the wall with ladders and remain undetected (perhaps due to a nearby forest)

They will then strike, and own your wall before you even know what's happened. Their concentrated and well trained troops will then hold what they've got while their comrades storm through the gate, or over the wall, and establish a bridge head.

Once their troops are assembled and in formation within your perimeter the war is essentially over. They will advance in formation, behind a shield wall, close with your defenders, and slaughter them. A few legions fighting in a determined and organized fashion will wipe out an undisciplined force 10 times bigger without issues.

Note: it's important to understand that a force will lose cohesion, and people will start running away long before most of them are dead. It's estimated that in medieval battles fewer than 30% of the troops would die on the battlefield. The rest would simply break ranks and run away when things started going south. Romans are a notable exception because their forces were highly disciplined troops, trained to fight together, and obey their chain of command. That sort of discipline was essentially unique before them, and wasn't really encountered again until much, much later.

I doubt a long siege with bombardments, and fancy siege machines would even take place.

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    $\begingroup$ A very informative assessment. Although I'm not sure whether or not your scale on population is too severe. This kingdom is basically 1/5th the size of Germany with 1/80th the population. Granted there'd still be overpopulation and crowding and problems, but that would stem from them not having nearly the manpower or tech to utilize that land, leaving a very large swarth of it field and farm and population centers pretty crammed. Disease, starvation, crowding, they'd all be pretty big issues inside the bird cage. $\endgroup$ – hankhoward Nov 15 '16 at 16:11
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    $\begingroup$ The question specifically states there are no gates. $\endgroup$ – user1975 Nov 15 '16 at 20:58
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    $\begingroup$ @Taemyr I'd say it's kind of obvious. If you can build a wall, you can scale it - otherwise, how would you make it so high in the first place? Scaling it under battle conditions is of course trickier, but don't forget how long the wall is, and how few defenders there are - there will be no battle, really. Twenty people can't hold a kilometer of a wall. That's why Rome had to keep pushing into the "barbarian" territory, after all - it was cheaper and easier to keep pushing them back (and recruit client states) than manning a stationary wall. Still, mining would probably be better than scaling. $\endgroup$ – Luaan Nov 16 '16 at 11:04
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    $\begingroup$ @hankhoward TBH I'm not sure where you get your numbers from. 156km diameter is around 10,000 sq miles for a square or 7,700 sq miles for a circle. I can't really imagine a figure that would give 24,000 sq miles with that diameter. So it's not 1/5th of Germany, by far, more like 1/15 or 1/20. And it's not 1/80 population (don't round 1,7 to 1...), it's actually 1/50th. So you can immediately see the problem - you have a population density only 2-3 times lower than a highly developed modern first world nation in ancient times. It's not even rural by most modern standards! $\endgroup$ – Ordous Nov 16 '16 at 13:11
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    $\begingroup$ @Ordous And a big part of the fallout after the fall of Rome was that the rich food supply at very low prices (Rome was quite imperialist) was lost. Lots of people dead, and most had to turn from crafts and other higher tier economies back to agriculture, while still being under considerable threat from the "barbarians" (and others that came later). How do you feed an army when you can barely feed yourself? The brilliant answer was feudalism. It took time for technology and investment to pay off before Europe could comfortably sustain itself. Half of Europe was forest or swamp, and cold. $\endgroup$ – Luaan Nov 16 '16 at 17:12
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The point of a siege is to cut a city or castle off from the outside world until they surrender or die of starvation. Since this is an entire hermit country, the classic idea of a siege is meaningless, what you're looking for is how to assault the kingdom.

With your now reduced army of 11,500, you have ~23 men per kilometre of your 490km wall. Not too bad for a patrol, but what are you going to do if I just roll my covered battering ram up to your wall and start cutting through? What if I started doing this in 2,3,4 places at once? As soon as you start focusing your defences on one point in the wall, you create weaknesses elsewhere and I can afford to put a couple of legions up against a kingdom of that size.

49 meters is a really big wall, too high to bother going over when you can go through. A wall is a built thing, given enough time it can also be dismantled and you don't have enough men to guard a wall that long against a superior force. Also I'm damaging the outside of your wall, you need to get men to the outside to repair it, which means dropping your guard and exposing civilians to danger.

Various other siege techniques would also still work, digging under and collapsing a tower for example. Prop the tunnel with wood so it doesn't collapse on your workers, then pull out the people and burn the supports when you're ready.

Given cannon, it's a matter of where you place them but it could well hold. With such superior firepower the wall is less relevant, what you want to do is meet a turtle on the open field and just lay into it with the cannon. Roman defensive formations were good for the enemies they had, but not against cannonballs. The problem however, is once the Romans have seen cannon, they're likely to come back with something similar on the next round.

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    $\begingroup$ It's a double-edged blade. On one hand, the watch patrol needn't be from soldiers, and a complicated messaging+Coordinated patrol system would work wonders, but not prevent say a dozen 50m+ Ladders from pouring a few hundred or thousand in. Still, going over the wall that way is like Hitler invading London one parachute drop at a time- intimation follows, much like the Blitz, impractical. A 14m x 49m thick wall 778 KM wide won't collapse easily. Digging under the wall and sending troops marching in is similar to climbing over it, but yet slightly more practical and much harder to detect. $\endgroup$ – hankhoward Nov 15 '16 at 17:25
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    $\begingroup$ Cutting and tearing through is definitely the most effective way Romans could barge in. However a 14m thick wall might take a bit to dig your way in, especially with Crossbows attempting to impede. They'd likely do it with slaves and turn it into a battle of attrition, i.e. how much fodder are they willing to risk dying to finally break in an entry point. Resources are in their ballpark for this and it's feasible the answer is simply "Enough." $\endgroup$ – hankhoward Nov 15 '16 at 17:31
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    $\begingroup$ @RB. You're probably going to have guard posts, bells on ropes, signal fires and all the works, especially if you're expecting guests. It's as much about how you focus your defensive troops once trouble arrives while maintaining a watch on the rest of the wall. $\endgroup$ – Separatrix Nov 15 '16 at 21:22
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    $\begingroup$ @hankhoward Actually, a bigger wall collapses much easier than a smaller wall. Masonry is very strong at compression, but very weak at tension and torsion - you're basically using the walls own weight against it. You don't even have to mine all the way through, so even thickness doesn't help. Cannons actually do pretty much the same thing - make the upper portions lose support, which tears the whole wall down. That's why cannons resulted in walls being lower - it makes them far more resistant to cannon fire. And Romans were quite skilled at mining in much harder conditions :) $\endgroup$ – Luaan Nov 16 '16 at 11:10
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    $\begingroup$ @hankhoward "watch patrol needn't be from soldiers" is strictly false - any person that is not working in agriculture but is required for this patrol duty is a "soldier" in the economic sense, he counts against the size of army that your country can sustain. $\endgroup$ – Peteris Nov 16 '16 at 12:09
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Brushing aside the realism of the kingdom and its wall, let's go on to tactics.

The real logistical problem is not so much the wall, but how to move troops and supplies in and out of the territory once the wall is neutralized. Without any gates, there's no way to effectively do this without tearing down the wall. So, as the Roman commander, my mission is to tear down that wall Mr. hankhoward.

The simple way of doing it would be to scout a way to undermine sections of the wall, and create a nice, big gap in it. A wall that big needs a lot of support, so destroying it from the foundations seems easiest. Let's assume your engineers thought of that, and sunk the foundations deep into the bedrock, or used some other methods to prevent undermining.

Using the mighty engineering knowledge that Rome was excellent at, my scouts would identify the weakest portions of the wall, and line up ever onager, ballista and trebuchet I could manage and hammer that wall for as long as it takes to make a hole. Doesn't matter how thick your wall is, eventually it will fail to that assault.

I don't like the dirt ramp option, but it is an option. The reason I don't like it is that it still makes transport of munitions hard. If I have a nice, big, hole in the wall, I can move my equipment through easily, but if I have to move everything up a ramp and over it, it becomes more difficult. Not to mention that I want Roman citizens to eventually colonize and trade in this land.

EDITS: A rule of cool option would be to set up a bunch of scorpions or other ballista-like siege engines and mount ropes to them. Run a bunch of legionnaires up to the walls, support them with archers to keep the enemy's heads down, and fire ropes over the walls. Legionaries climb up the ropes and take the wall. You could also do this with siege towers, ramps, or anything else, but the idea of firing ropes over the walls just seems cool (provided you could find a way to properly anchor them and keep them from getting cut). You could combine this with a distracting attack to keep the defenders concentrated in one area so your ropes don't get cut.

I also would like to see the option mentioned in another answer where the romans divert a river into the wall to undermine it. The best part about that plan is that it is relatively low maintenance once started (it just takes a while).

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    $\begingroup$ Citizens and trade aren't really a big of a deal - once you secure the wall, you can dismantle it brick-by-brick from the top. It would be much faster and easier than building the wall in the first place, so unless it was built using magic, deconstruction it would be a breeze for the skilled roman engineers. And you'd get tons of great construction material for your new citizens :) In fact, that's a major reason why so few walls survived to this day - once they stopped being manned and maintained, locals reused them to build cozy stone houses. Why quarry when you have stone/bricks ready? $\endgroup$ – Luaan Nov 16 '16 at 11:14
  • $\begingroup$ @Luaan two birds with one stone. Why use a siege method that will mean I'll have to dismantle part of the wall later. The logistics of the army are a bigger concern, especially if supplies inside the wall are only enough to sustain the large population within. $\endgroup$ – Marshall Tigerus Nov 16 '16 at 14:21
  • $\begingroup$ Surely a land that can comfortably supply more than a million civilians would not care much about another 10-20 thousand :) And historically, armies didn't care about keeping plenty food for the locals either - there's a huge difference between "comfortable daily food intake" and "bare minimum that will get you through the day while still allowing you to work well". $\endgroup$ – Luaan Nov 16 '16 at 17:20
  • $\begingroup$ There was a question about the population numbers and sustainability of agriculture in other answers, and armies like the British earned a lot of good will by paying for what they took and being careful how they took it. And if the Romans are trying to incorporate this territory into the Empire, they may well care about the health and well being of the people they are conquering. $\endgroup$ – Marshall Tigerus Nov 16 '16 at 17:48
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One idea that hasn't been mentioned yet:
Plant trees. Specifically, quick growing plants like pine. Plant them densely, right up to the base of the wall, or close enough. Start by planting a dense ring just out of crossbow range, wait till the saplings have grown a bit, so as to give cover, then plant another ring further forward, wait for them to grow... until you reach the walls, in ~80 years.
In a few years, they'll grow high enough to cover people moving on the ground. The defenders will either have to come down to clear the place by hand, and get picked off by your patrols, as they approach the treeline in the open, or give up their major offensive advantage: crossbow bolts can't very well hit what crossbowmen can't see. Unless, of course, they get a very dry summer and try to set the forest alight, then risk suffocation, as the smoke pours over the wall.
Now that you've got cover, you can do as you please to the wall, whether you divert the nearest river to play Ents and Orthanc, or dig under it, or build hundreds of rams from trees on the outer radius of the tree ring,or have ballistae/catapaults targeting the wall from under the tree cover, or even simply sneak long ladders close to the wall and raise them at night to infiltrate the country or just seize the wall.
The dumb part of this kind of defence is that you only need to capture the wall, and you get strategic command of the country. The inner side of the wall must have stairs for access, otherwise they'll never be able to reinforce the defences, and the invaders can simply pick the reinforcements off as they stand around helplessly. Hold the wall, and you can simply have your people walk down into the country.

Regarding the advantages of muskets and cannon, they might actually prove a liability. Why? Simply because of resource requirements. You need sulphur, carbon and saltpetre for gunpowder, lead for shot and steel or cast iron for muskets and cannon. Then you need heat sources, either coal or firewood for casting balls (both shot and cannon-)and tubes (cannon and musket barrel). You need lifts and winches to get the cannon and ball up the wall, shelters to keep your powder dry and safe from fire. All these will need to be extracted/manufactured, which requires manpower, or imported. And you can't import because some bright spark decided to wall up your seaport as well.
That leaves you with extracting/mining and manufacture, drawing people away from useful work like growing crops, repairing infrastructure or simply manning the wall. And it's only useful until the Romans figure out that the strange powder is very inflammable and the magic weapons are useless without it. Of course, if by then you've managed to convince them that you have Jupiter's favour, you're safe. Otherwise, they'll take potshots at the top of the wall with flaming bags of plant matter soaked in oil, and hope they manage to hit one or your caches. And you'll probably run out of powder before they run out of dry grass and leaves.

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  • $\begingroup$ Out of curiosity, why would said reinforcements just stand around helplessly? Rather than attempt to incapacitate any forces that made it up , under, or inside the walls before they have time to regroup, get set up and start sending legion formations up on the inside. Although the method to ascend/descend tilts the scale, if it's stairs it favors Romans since crossbows would be useless in an enclosed space. If it's ropes or latters, it favors natives, considering the difficulty of climb up/down while having arrows swarm down. $\endgroup$ – hankhoward Nov 16 '16 at 11:45
  • $\begingroup$ And for your entire comment on the resources needed, it's presumed that they've already mined, processed and utilized gunpowder, canons and the tech+resources necessary for Cannon fire and partitioned it off very carefully (The government inside the walls keeps tight control of all materials and mines, production, industrial capability, ect.) It's within 4500 square miles of the German area so it's not unfeasible to have those resources. $\endgroup$ – hankhoward Nov 16 '16 at 11:51
  • $\begingroup$ @hankhoward Comment1: If the defenders don't have stairs and the Romans have taken the top of the wall, there is nothing they can do but stand around. The Romans will use ladders on the outer side to reinforce their numbers, while their archers will pick off any natives that come too close. Regarding Comment 2: They may have the resources, but can they replace them as fast as they get used? Muskets and cannon aren't exactly accurate and the invaders have numerical advantage. Increasing production of war materials means tasking more people to them. That's less people for other essentials. $\endgroup$ – nzaman Nov 16 '16 at 12:11
  • $\begingroup$ "Specifically, quick growing plants like pine. Plant them densely, right up to the base of the wall, or close enough." You assume a lack of oil and ways to lit a fire or you suppose the Romans managed to find a fire-proof tree species. Maybe they can bring some quick growing grasses? $\endgroup$ – Adrian Colomitchi Nov 19 '16 at 12:49
  • $\begingroup$ @AdrianColomitchi: I answered that, if you'll note, "...and try to set the forest alight, then risk suffocation, as the smoke pours over the wall." $\endgroup$ – nzaman Nov 19 '16 at 19:27
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500'000 m of wall, with 11,250 soldiers to guard it.

Account for sleep, training, and other duties, and at any time you'll have no more than 3,000 soldiers to guard that 500'000 m wall. That's 3 soldiers every 500 meters. Enough to let you know when the wall's been breached, but nowhere near enough to even consider defending it.

Walk up to the wall, demand a surrender, done.


You can climb it, launch a mass of ropes, tunnel it, use a spy, attack through the water exit/entry points, poison the water supply, etc. Crossbows with their low rate of fire are quite useless if we look at the numbers: 3 soldiers firing at a 500m wide mass of soldiers, at 1 bolt/minute, while the soldiers climb the wall for 2-5 minutes ... They'd be better off either trying to cut the ropes (3 people vs 400 ropes, good luck), or more realistically, running away.

Sure there are some more elaborate ways to attack, but since the army can strike anywhere at any time, anything other than either a night attack using lots of ropes, or a spy / bribed guard, seems like a waste of time and resources.

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  • $\begingroup$ Considering the topography of this particular location (From a quick look at where they'd be located), poisoning the water would be difficult to impossible with some small exceptions. Water and the rivers flows south to south-west on the map in this location generally so the rivers would be going outwards away from the wall, not in. Unless you can poison the entire ocean or have 100% expansion of all northern territories above the Kingdom. $\endgroup$ – hankhoward Nov 16 '16 at 13:49
  • $\begingroup$ @hankhoward I'm not sure where exactly you want this to be, but pretty much everywhere in Belgium and Germany water is flowing in. Throwing in a couple dead animals won't kill everyone, it is merely an inconvenience. But that inconvenience might be enough for them to give in to whatever demands the Romans have. $\endgroup$ – Peter Nov 16 '16 at 16:59
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  1. First dig a tunnel. Getting a large army in by rope is not a option. Since the Romans can't fly, the only option I see is a tunnel.

  2. Bribe the guards. The Mongols were able to get past the Chinese defenses and into mainland China by simply bribing the guards. The Romans could use a similar tactic.

  3. Live off the land. One of the things that made the Mongols so formidable was that they had no supply line. They took everything they needed from their enemies. َ

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  • $\begingroup$ This is, an interesting tactic I never even considered. Simple sabotage. Thank you for this one! $\endgroup$ – hankhoward Nov 16 '16 at 14:37
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Dig a Tunnel

You do not list how deep the wall goes in land (only 8m in water) so I am going to assume 8m in land too. The army could build a stone wall to protect themselves from attacks from inside the wall, and behind the wall tunnel into the ground. The tunnel would go under the wall and up into the city. Troops would then move in at night through the tunnel and attack.

Multiple tunnels could be used in order to create a larger attack. Also tunneling directly under the wall could cause it to possibly collapse enough for ground level entry.

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Trebuchets with fire and disease. Trebs could easily put things over the walls and splatter burning oil on sections of the wall to isolate it from reinforcement.

Attacking is not about matching numbers of force, that is game stuff. It is about creating the most massive force imbalance at some point you possibly can, then exploiting that imbalance.

Expanding on the treb with burning oil example, some siege towers 50 meters from the wall (crossbows are not powerful that far away with longbowmen(they reload much faster) to pick off anyone up there. The force imbalance is the locational attrition. The city-state will be getting no reinforcements and every person on the wall that picked off is one less to respond... and one more to convince the rest of the citizens that resistance is dumb.

The trebuchets keep up the fire and disease bombardments. Two or three towers are built from wood ( a week at most) to allow surveillance of the city as well as spotting for the trebuchet corps' efforts. Once the wall is cleared in one section then build fires at the base every day. Masonry is not heat resistant forever. A section of the wall will come down and the Romans suffer few if any casualties. The wall has served to be a force imbalancer... on the side of the Romans because it prevented the city dwellers from ever using their more powerful weapons in a useful way. City had really stupid leaders and generals.

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    $\begingroup$ Depending on what level of tech the Kindom possesses, trebuchets and catapults alone won't be sufficient. Especially if they possess a similar level of bombardment technology to pelt and rain down at any artillery back. And I have doubts continuous arson would be reasonable for a wall of this magnitude. If I were a general of this city, I'd be more terrified of Rome building a monstrosity like the Greek Helepolis, a 40m Siege tower carried by a crew of 3400 men, one of the strongest ever built (305 BC). If Rome produced a few of those bombardment would suddenly be the least of their worries. $\endgroup$ – hankhoward Nov 16 '16 at 5:36
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    $\begingroup$ "Trebuchets with fire and disease." I just imagined an army using trebuchets to fire rotting and sick corpses and animals at their enemy. If only they could make it safe for themselves it would be a pretty interesting tactic also from the psychological point of view, almost a kind of medieval leaflet bomb. $\endgroup$ – Maurycy Nov 16 '16 at 10:09
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    $\begingroup$ @Maurycy it was done throughout the middle ages. Most of siege warfare was waiting for one side or the other to succumb to disease. Siegers tended to win because they had more access to supplies and could get reinforcements. $\endgroup$ – Marshall Tigerus Nov 16 '16 at 14:28
  • $\begingroup$ @MarshallTigerus Can you point me to some names of sieges where this tactic was used? I'd really like to read more about it! $\endgroup$ – Maurycy Nov 16 '16 at 14:46
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    $\begingroup$ Siege of Kaffa/Caffa is one. Also en.wikipedia.org/wiki/History_of_biological_warfare Germ warfare is apparently older than Christianity. $\endgroup$ – Marshall Tigerus Nov 16 '16 at 14:48
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Others have noted Roman military capabilities, which certainly are enough for the job. Instead of repeating those I will point out a critical difference between Roman and modern psyche.

It is hard for us to understand Romans because they, in their height, looked not at Rome's power as a undisputed permanent fact. We often say that the Romans overengineered their solutions, but that's only our standpoint. One born in a world where making a 3-year industrial investment is nearly unheard of for most parts of our current world. Sure we can build some infrastructures longer. But in essence we are living in a world where we can not justify many mega projects and instead keep maintaining ones that are well past their prime, because we look at the world one quarter, maybe a year at the time.

Generations before us did not have this problem! China, Egypt and Rome were built to last. Romans were not afraid to do investments that spanned generations. This spells doom to your fantasy kingdom. The Romans could afford to spend years and decades on this siege.

It is nearly impossible for us to understand what it means to be able to focus so much in human resources. It is a totally different ballgame. On top of this the fantasy kingdom's defenses are not beyond the capabilities of Roman engineering.

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Many good, long and detailed answers. I'll try to make it short and answer only the question you asked:

Rome would have absolutely no problem at all to defeat that wall. They proved this in the Siege of Masada (a mountain fortress), where the Roman army spent months constructing a huge ramp.

Rome would also absolutely go and do this, no matter the cost, time or effort. Rome, especially at the height of its power, was very much interested in keeping its reputation as part of psychological warfare, and part of that was that Rome always wins a siege in the end.

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Assuming your kingdom shape is a circle, 156 km diameter means that the circumference is:

490,000 m

So from a logical point your fantasy kingdom have enough people to keep the wall guarded: It needs just one person every 100 meters or so, so just 4900 people are needed and the population is over the million so just 1-2% of population is needed for guarding the wall (well, in reality triple that value assuming each person works only eight hours a day).

  • Now to put a ladder and climb a wall of fifty meters. You just need 1 minute.
  • The world record running speed is just slightly above ten meters/second
  • In the time a single Roman climb up a wood ladder, only six guardians can come to help, leaving 600 meters of wall unguarded.

If you mass up enough Romans they can just climb up and start invading.

  • Assume you put one Roman every five meters
  • Assume a defender can throw away one ladder every three seconds

Then you just need 120 Romans to get a bunch of them up to the wall (in reality that only if six defenders can gather instantaneously, which is impossible). If you have 130 Romans you can get for sure ten Romans on the wall while defenders are just six.

Advancement in bow technology is not relevant, because if Romans are so few you cannot kill that many of them (bows are effective against tightly packed enemies).

Again assume defenders have perfect aim and can shoot one arrow every three seconds, or they shoot or they remove a ladder so the total number of Romans to get up on wall do not change, but you need some extra Romans to get near the wall:

  • Bows have 200 meters range? (100 meters for perfect shoot, and 100-200 for 50% shoots)
  • Romans run at five meters/second
  • A Roman takes 200/5 = 40 seconds to reach the wall
  • Assume 42 seconds, that's 14 extra shots with perfect aim, and 11 with normal aim
  • since there are six defenders you have 66 extra shots

In the end you just need 200 Romans to get ten Romans up the wall. If you do that in many places at the same time, you conquer the wall. Or you can just amass an army along ten kms of walls to get some hundreds Romans on walls.

Now, is a fifty m ladder feasible? Probably not.

But you don't need wooden ladders; just give enough bows to Romans. Do they have shorter range due to lower technology and height difference? No problem, you just need some time to move more Romans near the wall. Defenders have to die due to arrows or to retreat to keep safety range, in both cases leaving a piece of wall unguarded so that Romans can start demolishing it or climbing it.

Also, I would not be surprised if Romans would be able to put rope ladders (throwing rampling hooks) working within 1-2 hours and within few days build a more stable way to cross the wall.

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The Romans were a formidable military power, but they weren't invincible. There was a stalemate against Persia and Armenia, and they also were hemmed by Aksum (controlling both banks of the Red Sea).

They also stopped conquering land when they thought it wasn't worth the effort (e.g., they never conquered Scottland or Ireland, and they gave up on conquering Germany).

The walls are technically not a problem for the Romans. The size of the army is a real problem, and the crossbows are another problem: They will cause heavy losses among the Roman legions. Probably, the Romans would acquire the crossbow technology sooner or later (by espionage, treachery, or reverse engineering from artefacts found on the battleground).

Still the army of the fantasy kingdom is much to large for the Romans to overcome. They can hope the the fantasy kingdom does not try to conquer Rome!

EDIT: I see that the army was decimated by a factor of 10: Now it is feasible for the Romans to win, but yet very hard.

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Such a big wall will be impossible to defend with only 11,000 people. If the diameter is 156 km, the circumference is 490 km, a diameter of 250 km gives us circumference of 785 km.

The smartest thing would be to put most of your army at the center so that they are "only" 78 km (or 125 km) away from any point on the wall.

Say the Romans attack the wall and are trying to scale it. Your wall guard has to travel 78 km to notify the army, then the reinforcements have to travel back 78 km to the wall.

The Romans have much superior numbers; they can launch simultaneous attacks, scale up the wall and establish a beach head for the rest to follow. At that point the question is if the superior technology the defenders have gives them enough edge over the Romans. I would say no. Medieval firearms were under-powered, inaccurate and not very reliable.

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